Skeletogenic Expression of <i>Integrin Alpha</i> , <i>Talin</i> and <i>Npnt</i> Genes and Npnt Role in Sea Urchin Skeletogenesis

Biomineralization, the formation of mineralized tissues like skeletons and shells, is an essential developmental process in diverged phyla. Vertebrates' biomineralization involves the secretion of specialized extracellular matrix (ECM) proteins and the formation of Integrin-based focal adhesions, yet less is known about the role of such factors in invertebrates. A recent study has shown that focal adhesions form around the calcite spicule of the sea urchin larva, however, the skeletogenic expression and role of adhesion related proteins in this system are understudied. Here, we identified a set of ECM and adhesion genes that show enriched expression in the sea urchin skeletogenic cells and studied the role of the ECM protein, Npnt, in Paracentrotus lividus. The integrin alpha proteins, Pl-Ahi, Pl-Aji, Pl-Api, and the Pl-Talin protein are highly conserved between sea urchin and humans and the expression of these genes is enriched in the skeletogenic cells during early skeletogenesis. Pl-npnt is expressed specifically in skeletogenic cells throughout skeletogenesis and requires Vascular Endothelial Growth Factor (VEGF) signaling for its maintenance. Genetic perturbations of Pl-npnt result in skeletal defects, including reduced length of skeletal rods, ectopic spicule formation and branching, while skeletogenic cell migration remained unaffected. The activation of focal adhesion kinase (FAK) around the spicules is independent of Pl-Npnt activity in agreement with the loss of Integrin binding site in the sea urchin Npnt protein. Our findings set the stage for further analyses of ECM and adhesion-mediated mechanisms that drive sea urchin biomineralization, and most likely participate in skeletal development across metazoans.